1. Field of the Invention
The present invention relates to an optical disc having wobbled tracks, and an apparatus (recording/reproduction apparatus, mastering apparatus, or the like) or method using this optical disc.
2. Description of the Related Art
As is well known, in recent years, optical discs having a single-layer/single-sided size of 4.7 GB are commercially available as those which can achieve high-density recording of information. For example, a DVD-ROM is available as a read-only optical disc, and a DVD-RAM, DVD-RW, DVD+RW, and the like are available as rewritable discs. Also, a DVD-R is available as a “write-once” optical disc.
On these optical discs, an information recording layer is formed on a transparent substrate, and information is recorded/reproduced by focusing a laser beam on that layer. As information recording/reproduction means for rewritable optical discs, an information recording layer of such optical disc has a guide groove called a “groove” Recording/reproduction of information on/from a rewritable optical disc is made along this guide groove. Furthermore, this optical disc is formed with physical addresses used to specify spatial positions where information is to be recorded/reproduced.
As formation means of the physical addresses, a DVD-RAM uses intermittent three-dimensional patterns called prepits on a substrate. By contrast, a +RW disc adopts groove wobble modulation (to be referred to as wobble modulation hereinafter) that radially slightly wobbles the guide groove. The physical addresses formed by wobble modulation can assure a broad recording area for user information (i.e., high format efficiency) and can easily take compatibility to read-only media, since they do not obstruct recording tracks (unlike the prepits).
As a prior art that forms physical addresses by wobble modulation, Jpn. Pat. Appln. KOKAI Publication No. 2002-279645 (FIGS. 4 and 5; paragraph Nos. 0029 and 0030) is known.
In this reference, physical addresses are formed by wobble phase modulation, and one address bit is made up of a modulation unit for four wobbles, and a monotone unit for 38 wobbles. More specifically, let + be +90° modulation, and − be −90° modulation. Then, wobbles modulated by ++−− and monotone wobbles +++ . . . +++ express “0”, and wobbles modulated by −−++ and monotone wobbles +++ . . . +++ express “1”. In order to read each address, the address start position must be detected, and 1 bit of a sync signal for this purpose is formed by 42 wobbles like an address bit.
Upon demodulating a phase-modulated wobble signal, the frequency band must be limited using, e.g., a band-pass filter to reduce the influence of disturbance noise other than the wobble signal. However, when a wobble signal is phase-modulated, a frequency in a frequency band different from that of the wobble signal is present at a phase change point. For this reason, at the phase change point (having a frequency component higher than the wobble signal frequency) of the wobble signal that has passed the band-pass filter, the amplitude attenuates depending on the filter characteristics. This amplitude attenuation becomes disturbance upon demodulation.
Therefore, in order to attain more accurate demodulation, modulation is preferably made to minimize occurrence of phase change points. However, in a conventional system that records physical addresses by wobble phase modulation, the wobble phase is inverted a plurality of times in one address bit so as to express the address symbol. When such modulation method is used, if external noise increases due to track pitch narrowing, demodulation errors are more likely to occur.